Method of reducing NOx emissions using a fluid-cooled injector

ABSTRACT

A method for reducing emissions of oxides of nitrogen from a combustion process using a temperature sensitive liquid reagent injected into a stream of exhaust gases from the combustion process and passing the exhaust gases and the reagent through a catalytic reactor which reduces the oxides of nitrogen in the presence of the reagent is disclosed. The steps of the method include providing an injector having an orifice for atomizing the liquid reagent; positioning a portion of the injector having the orifice within the stream of exhaust gases; cooling the injector by continuously circulating the reagent therethrough, thereby keeping both the injector and the reagent within the injector below a critical temperature at which the reagent will solidify; and injecting a portion of the reagent into the exhaust stream upstream of the reactor.

RELATED APPLICATION

[0001] This is a divisional application based on U.S. application Ser.No. 09/164,304, filed Oct. 1, 1998.

FIELD OF INVENTION

[0002] This invention relates to methods for reducing NO_(x) emissionsfrom internal combustion engines and especially to methods usingfluid-cooled injectors wherein the fluid is a liquid reagent and aportion of the reagent is injected as an atomized liquid reagent intothe exhaust gas stream of an internal combustion engine.

BACKGROUND OF INVENTION

[0003] Improved fuel efficiency for vehicles having internal combustionengines can be achieved by using diesel engines or gasoline enginesoperated with an excess of oxygen over the amount necessary for completecombustion of the fuel. Such engines are said to run “lean” or on a“lean mixture”. The increase in fuel economy, however, is offset byundesired pollution emissions, specifically in the form of oxides ofnitrogen (NO_(x)).

[0004] One method used to reduce NO_(x) emissions from internalcombustion engines is known as selective catalytic reduction (SCR). SCR,when used, for example, to reduce NO_(x) emissions from a diesel engine,involves injecting an atomized reagent into the exhaust stream of theengine in relation to one or more selected engine operationalparameters, such as exhaust gas temperature, engine rpm or engine loadas measured by engine fuel flow, turbo boost pressure or exhaust NO_(x)mass flow. The reagent/exhaust gas mixture is passed through a reactorcontaining a catalyst, such as, for example, activated carbon or metals,such as platinum, vanadium or tungsten, which are capable of reducingthe NO_(x) concentration in the presence of the reagent. An SCR systemof this type is disclosed in U.S. patent application Ser. No.08/831,209, hereby incorporated by reference.

[0005] An aqueous solution of urea is known to be an effective reagentin SCR systems for diesel engines but suffers several disadvantages.Urea is highly corrosive and tends to attack mechanical components ofthe SCR system, such as the injectors used to inject the urea mixtureinto the exhaust gas stream. Urea also tends to solidify upon prolongedexposure to elevated temperatures, such as encountered in diesel exhaustsystems. Solidified urea tends to accumulate in the narrow passagewaysand orifice openings typically found in injectors. The solidified ureafouls moving parts of the injector and clogs any openings, thus,rendering the injector unusable.

[0006] Furthermore, if the urea mixture is not finely atomized, ureadeposits will form in the catalytic reactor, inhibiting the action ofthe catalyst and thereby reducing the SCR system effectiveness. Highinjection pressures are one way of dealing with the problem ofinsufficient atomization of the urea mixture, but high injectionpressures often result in over-penetration of the injector spray plumeinto the exhaust stream, causing the plume to impinge on the innersurface of the exhaust pipe opposite the injector. Over-penetrationleads to inefficient use of the urea mixture and reduces the range overwhich the vehicle can operate with reduced NO_(x) emissions. Like fuelfor the vehicle, only a finite amount of aqueous urea can be carried andwhat is carried should be used efficiently to maximize vehicle range andreduce the need for frequent fill ups of the reagent.

[0007] Additionally, aqueous urea is a poor lubricant. Thischaracteristic adversely affects moving parts within the injector andrequires that special fits, clearances and tolerances be employedbetween relatively moving parts within an injector.

SUMMARY AND OBJECTS OF INVENTION

[0008] The method according to the invention concerns reducing emissionsof oxides of nitrogen from a combustion process using a temperaturesensitive liquid reagent injected into a stream of exhaust gases fromthe combustion process and passing the exhaust gases and the reagentthrough a catalytic reactor which reduces the oxides of nitrogen in thepresence of the reagent. The steps of the method include providing aninjector having an orifice for atomizing the liquid reagent; positioninga portion of the injector having the orifice within the stream ofexhaust gases; cooling the injector by continuously circulating thereagent therethrough, thereby keeping both the injector and the reagentwithin the injector below a critical temperature at which the reagentwill solidify; and injecting a portion of the reagent into the exhauststream upstream of the reactor.

[0009] The reagent is preferably an aqueous urea solution which isinjected into the stream of exhaust gases in proportion to selectedengine operating parameters. Preferably the urea has a concentrationbetween about 25% and about 35%. The reagent is circulated continuouslyat a rate which will keep its temperature below about 140° C. andpreferably below about 95° C.

[0010] The invention also provides an injector for delivery of a fluidinto a stream of hot gas, the injector being designed to operateeffectively with a corrosive, temperature-sensitive reagent, such asaqueous urea. When used in the method according to the invention forreducing NO_(x) emissions, the injector is mounted on an exhaust conduitof an internal combustion engine where it injects the reagent into theexhaust gas stream.

[0011] The injector comprises a valve and a means for actuating thevalve between a closed position and an open position. Acceptableactuating means include, for example, a solenoid-type actuator.Preferably, the components of the valve exposed to extreme heat orcorrosive reagents like urea are made of a corrosion resistant materialsuch as stainless steel.

[0012] The valve includes an orifice through which the reagent isexpelled when the valve is in the open position. Regardless of the stateof the valve (i.e., open or closed), the reagent is continuouslycirculated through it when the system is in operation, at least aportion of the circulating reagent being expelled when the valve isopened. The circulation of the reagent cools the valve and minimizes thedwell time of the reagent within the valve, thereby minimizing exposureof the reagent to heat and the creation of urea deposits. Thus, aqueousurea, for example, can be effectively used with such an injector withoutthe characteristic fouling and clogging of the injector. Meansindependent of the valve actuating means are provided for continuouslycirculating the reagent through the valve, as described in detail below.

[0013] Preferably, the valve comprises a valve body which has anelongated cylindrical chamber therein in fluid communication with theorifice. A valve seat is positioned within the chamber surrounding theorifice. An elongated valve plunger is slidably mounted within thechamber. One end of the plunger is sealingly interengagable with thevalve seat to close the orifice. The plunger is connected with theactuating means and is movable from the closed position where theplunger end sealingly engages the valve seat and the open position wherethe plunger end is removed from sealing interengagement with the valveseat to open the orifice.

[0014] The means for independently circulating fluid through the valvecomprises a portion of the plunger which is arranged adjacent to theplunger end. This portion of the plunger has a diameter less than thechamber diameter and forms an annular fluid space or passageway withinthe valve adjacent to the valve seat and the orifice. The annularpassageway, thus, allows for both the continuous circulation of fluidthrough the valve and the expelling of a portion of the fluid throughthe orifice when the valve is in the open position.

[0015] Preferably, the independent fluid circulating means furthercomprises a fluid inlet and a fluid outlet arranged within the valvebody in fluid communication with the annular passageway. Fluid, such asthe aqueous urea reagent, is supplied from a reservoir and flows intothe valve through the inlet, continues through the annular passagewayand exits the valve via the outlet, thereby cooling the injector. Whenthe valve is opened by the actuator, the valve plunger is moved to theopen position, and a portion of the fluid is expelled from the chamberthrough the orifice.

[0016] In order to provide additional heat protection for the injector,a heat shield is preferably interposed between the valve and the streamof hot gas. The heat shield has an aperture which is aligned with theorifice. The heat shield aperture allows fluid expelled from the valveto pass through the heat shield and into the hot gas stream. The heatshield preferably comprises a metal plate and a layer of insulatingmaterial interposed between the plate and the valve. The heat shieldaperture passes through both the layer of insulating material, as wellas the metal plate.

[0017] To improve atomization of liquid reagents, especially atrelatively low injection pressures, an atomizing hook is preferablymounted on the valve. The atomizing hook has an end surface which ispositioned in a spaced apart relation with the orifice. Liquid reagentexpelled through the orifice impinges on the hook end surface wherefurther atomization of the reagent occurs. The shape and position of thehook end surface directly affect the dispersion characteristics of theinjected reagent.

[0018] It is an object of the invention to provide a method for reducingNO_(x) emissions from a combustion process by injecting a temperaturesensitive liquid reagent into a stream of exhaust gases from thecombustion process.

[0019] It is another object of the invention to provide a method whichuses urea as the liquid reagent.

[0020] It is still another object of the invention to provide a methodwhich uses aqueous urea at relatively high concentrations.

[0021] It is yet another object of the invention to provide a methodwhich uses an injector to inject the reagent into the exhaust stream.

[0022] It is again another object of the invention to provide a methodwherein the reagent is continuously circulated through the injector tokeep both the reagent and the injector below a temperature at which theurea will solidify.

[0023] It is an object of the invention to provide an injector forinjecting a fluid into a stream of hot gas.

[0024] It is another object of the invention to provide an injectoruseable with corrosive liquids such as aqueous urea.

[0025] It is yet another object of the invention to provide an injectorin which aqueous urea will not solidify when the injector is exposed toheat.

[0026] It is still another object of the invention to provide aninjector which achieves fine atomization of liquid reagents atrelatively low injection pressures.

[0027] It is a further object of the invention to provide an injectorwherein a portion of the fluid being injected is also continuouslycirculated through the injector to cool the injector.

[0028] It is yet a further object of the invention to provide aninjector wherein the dwell time of the fluid within the injector isminimized.

[0029] It is still a further object of the invention to provide aninjector useable in a pollution control system for reducing NO_(x)emissions of internal combustion engines.

[0030] These and other objects will become apparent from a considerationof the following drawings and detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 shows a schematic diagram of a pollution emission controlsystem using an injector according to the invention;

[0032]FIG. 2 shows a longitudinal cross-sectional view of an injectoraccording to the invention; and

[0033]FIG. 3 shows a side view of the valve body of the injectoraccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034]FIG. 1 illustrates a pollution control system as might be used toreduce NO_(x) emissions from the exhaust of a diesel engine 3. Thesystem includes an engine exhaust conduit 4 in fluid communication witha catalytic reactor 5, a reagent reservoir 6 holding reagent 7, acentral processing unit 8 and an injector 10. Injector 10 is mounted onexhaust conduit 4 and fed reagent, for example, a solution of aqueousurea via supply line 9 extending from reservoir 7 to the injector. Apump 11 is used to pump the reagent to the injector at a predeterminedpressure. Reagent 7 is circulated back to the reservoir via return line12, the circulation of the reagent being shown by the arrows 7 a.

[0035] In operation, signals 13, representing engine operationalparameters such as exhaust gas temperature, engine speed and fuel flowrate are monitored by central processing unit 8. In response to thesesignals and preprogrammed algorithms, central processing unit 8 sendscontrol signals 14 and 15 to injector 10 and pump 11 respectively, thecontrol signals commanding pump 11 to circulate reagent and injector 10to inject or cease injecting reagent into exhaust gases 16 within theexhaust conduit 4. The reagent is atomized upon injection into theconduit and forms a mixture with the exhaust gases. This mixture entersthe catalytic reactor 5 which contains a catalyst, such as activatedcarbon, or metals, such as platinum, tungsten or vanadium, which reducesNO_(x) in the exhaust gases in the presence of the reagent. The exhaustexits the conduit 4 and passes to the atmosphere.

[0036] During system operation, regardless of whether or not theinjector is releasing reagent into the exhaust gases 16, reagent 7 iscirculated continuously between the reservoir 6 and the injector 10 tocool the injector and minimize the dwell time of the reagent in theinjector so that the reagent remains cool. Continuous reagentcirculation is necessary for temperature-sensitive reagents, such asaqueous urea, which tend to solidify upon exposure to elevatedtemperatures of 300° C. to 650° C. as would be experienced in an engineexhaust system. It has been found to be important to keep the ureamixture below 140° C. and preferably in a lower operating range between5° C. and 95° C. to provide a margin of safety ensuring thatsolidification of the urea is prevented. Solidified urea, if allowed toform, would foul the moving parts and openings of the injector,eventually rendering the injector useless. In the case of a310-horsepower diesel engine with a baseline NO_(x) emissions level of 8grams/bHp-hr at full load, circulation rates of aqueous urea between 0.5gallons per minute and 0.75 gallons per minute through an injectoraccording to the invention have been found to effectively cool theaqueous urea and prevent solidification. It will be recognized that flowrates will depend on engine size and NO_(x) levels. It is an advantageof the invention that more concentrated solutions can be utilized, i.e.,25-35%, because throughout the system, the solution is not subject toconditions which would cause significant hydrolysis or solubilityproblems.

[0037]FIG. 2 shows a cross-sectional view of the preferred embodiment ofthe injector 10 according to the invention. The injector is shownmounted on an exhaust gas conduit 4, only partially depicted, incross-section. Injector 10 comprises a valve body 18 having an elongatedcylindrical chamber 20 disposed therein. Chamber 20 is in fluidcommunication with an orifice 22 which opens onto the exhaust gaseswithin conduit 4. Surrounding orifice 22 is a valve seat 24 which canhave any practical shape but is preferably conical. A valve member inthe form of an elongated valve plunger 26 is slidably mounted withinchamber 20. Valve plunger 26 has an end 28 formed to sealinglyinterengage valve seat 24, as seen in FIG. 2, thereby closing orifice 22from fluid communication with chamber 20.

[0038] Valve plunger 26 is movable within the chamber between the closedposition shown in FIG. 2 and an open position wherein end 28 is removedfrom sealing interengagement with valve seat 24. In the open position,orifice 22 is opened to fluid communication with chamber 20.

[0039] Together, the chamber 20 and the valve plunger 26 provide a meansfor circulating fluid, such as the reagent, through the valve forcooling the valve and for minimizing the dwell time of the reagentwithin the valve. The circulating means comprises an annular fluidpassageway 30 formed between the relatively larger inner diameter ofchamber 20 and the relatively smaller outer diameter of a section 32 ofthe valve plunger 26. Preferably, plunger section 32 is arrangedadjacent to plunger end 28 and close to valve seat 24 and orifice 22.Positioning fluid passageway 30 close to the orifice allows thecirculating fluid to directly cool an otherwise hot part of the valvebody most sensitive to the adverse effects of heat. Thus, for example,aqueous urea, when used with this cooled valve, will not solidifyanywhere within chamber 20. If allowed to solidify, the urea couldprevent plunger 26 from seating properly or could cause the plunger toseize in either the open or closed position and/or the orifice 22 couldbecome clogged. By directly cooling this region of the valve, however,the detrimental effects of elevated temperature on the reagent, themoving parts, and the openings of the valve are avoided.

[0040] As seen in FIG. 2, plunger 26 further comprises a guide section33 disposed adjacent to section 32 of the valve plunger. Guide section33 preferably has a polygonal cross-section formed by a plurality offlats 33 a intersecting at a plurality of corners 33 b. Flats 33 aprovide fluid circulation spaces adjacent to the chamber 20 and augmentthe cooling function of the fluid passageway 30. The flats also providespace for any debris formed within or brought into chamber 20 to washout of the chamber with the circulating fluid.

[0041] The corners 33 b of the guide section 33 provide a stabilizingand a guiding function for plunger 26. The corners are toleranced toride close to or in light contact with the wall of chamber 20 to providesupport points which guide the plunger within the chamber to ensureproper seating of plunger end 28.

[0042] Immediately above guide section 33 is a reduced circularcross-section 35 of plunger 26. Reduced section 35 provides an annularspace for fluid to flow into the chamber through an inlet, described indetail below. Above the reduced section is a circular guide section 37.Circular guide section 37 provides the main guiding function for slidingmotion of the plunger 26 within the chamber 20. The tolerance betweenthe circular guide section and the chamber is sufficient to allowrelative motion and lubrication of the plunger while still guiding theplunger's motion and forming a partial hydraulic seal between theplunger and the chamber.

[0043] Generally, the specific tolerances required at the varioussections between the valve plunger and the chamber will vary accordingto the operating temperature, operating pressure, the desired flow rateand circulation rate of the reagent, the tribological properties of thereagent and the materials chosen for the valve plunger and valve body.The tolerances for optimum injector performance are best obtainedexperimentally by a few field trials.

[0044] The cooling fluid is delivered to the annular fluid passageway 30through fluid inlet 34. Fluid inlet 34 is arranged within valve body 18in fluid communication with chamber 20 and is externally connected tosupply line 9 (FIG. 1). It is preferred that the fluid inlet bepositioned to deliver fluid to chamber 20 in a region removed from thevalve seat 24 adjacent to reduced section 25 and guide section 33, asshown in FIG. 2. Positioning the fluid inlet upstream from the seat, asshown, allows the fluid to contact valve plunger 26 over a substantiallength before it encounters the valve seat, thereby enhancing thecooling function of the fluid. Fluid, such as reagent 7, is pumped viapump 11 at a predetermined pressure into the fluid inlet 34 from whichit flows along valve plunger 26 into annular fluid passageway 30.

[0045] A fluid outlet 36 is provided to remove the fluid from theannular fluid passageway. Fluid outlet 36 is arranged within valve body18 in fluid communication with chamber 20. Preferably, fluid outlet 36is positioned as shown in FIG. 2 for removing fluid from chamber 20 inthe region of the valve seat 24. Fluid outlet 36 is externally connectedto return line 12 (FIG. 1), thus permitting the fluid (such as reagent7) to circulate from reservoir 6, through supply line 9, through fluidinlet 34, into annular fluid passageway 30, through fluid outlet 36,through return line 12 and back into reservoir 6. This circulation keepscritical regions of the valve body 18 cool and minimizes the dwell timeof the fluid in the injector.

[0046] When the valve plunger 26 is moved from the closed position,shown in FIG. 2, to an open position, plunger end 28 is removed fromsealing interengagement with seat 24. This action opens orifice 22 andallows at least a portion of the circulating fluid to be expelledthrough the orifice and into exhaust conduit 4. To effect the openingand closing of the orifice, actuating means are provided, preferably inthe form of solenoid 38 mounted atop valve body 18. Solenoid 38 has anarmature 40 connected to valve plunger 26. When the solenoid isenergized, the armature 40 is drawn upward, thereby sliding valveplunger 26 within chamber 20 from the closed position to the openposition. The solenoid would be energized, for example, in response to asignal 14 (see FIG. 1) from central processing unit 8, which decides,based upon sensor input signals 13 and its preprogrammed algorithms,when reagent is needed for effective selective catalytic reduction ofNO_(x) emissions in the exhaust stream.

[0047] As seen in FIG. 2, valve plunger 26 is biased in the closedposition by a biasing member, preferably in the form of a coil spring 42coaxially disposed about valve plunger 26. The valve plunger has ashoulder 44 which serves as a lower spring seat against which the springcan push to bias the valve plunger. An upper plate 46 is fixed to thevalve body 18 and serves as the upper spring seat, as well as a stop tolimit the upward travel of the valve plunger.

[0048] Spring 42 is located within a spring chamber 48 which is isolatedfrom chamber 20 by seal 50. Seal 50 is preferably made of carbonreinforced Teflon® or glass reinforced Teflon® and prevents anycorrosive reagent from entering the spring chamber and possiblyattacking or fouling the spring and the solenoid.

[0049] Injector 10 is shown mounted on exhaust conduit 4 by means ofsleeve 52 which is welded to an opening in the conduit by weldment 54.Preferably, valve body 18 has external threads 19 which engage matchinginternal threads 53 in sleeve 52 to attach the injector to the sleeve.In order to minimize conductive heat transfer between the sleeve and thevalve body, the external threads 19 are not continuous around thecircumference of valve body 18 but interrupted or discontinuous, as seenin FIG. 3. Preferably, the thread contact area is minimized by usingintermittent arcs of threads subtending angles on the order of 20°arranged circumferentially around valve body 18, with flat regions 21arranged between each thread arc. The flats have an across-the-flatdimension which is less than the root diameter of the thread on valvebody 18 and, therefore, make no contact with sleeve 52.

[0050] In the configuration shown, hot exhaust gases within the conduitare prevented from impinging directly upon the valve body 18 by theinterposition of a heat shield 56 between the valve body and the exhaustgases. Heat shield 56 includes an outer metal plate 58 and a layer ofinsulating material in the form of a thermal gasket 60 interposedbetween outer plate 58 and valve body 18. Preferably, outer plate 58 ismade of stainless steel to resist the corrosive environment within theexhaust conduit. Gasket 60 is preferably made of a flexible graphitefoil material whose low thermal conductivity serves to isolate valvebody 18 from outer plate 58, reducing conductive heat transfer to theinjector and thereby helping to keep the fluid circulating within thevalve cool.

[0051] Heat shield 56 surrounds the orifice 22 and has an aperture 62which passes through both the outer plate and the insulating thermalgasket and permits fluid expelled from the injector to pass through theheat shield and into the conduit. The heat shield has a substantiallyplanar surface which is preferably oriented perpendicular to the jet offluid expelled from the injector.

[0052] Further thermal protection for the injector is provided by aradiant heat reflector 70 seen edge on in FIG. 2. Reflector 70 ispreferably a round disc of polished aluminum having an outer diameter ofsufficient extent such that the surface 70 a of the disc blocks radiantheat transfer from exhaust conduit 4 to parts of the injector which havea direct line of sight to the conduit. The reflector has a centrallypositioned aperture 72 which fits around valve body 18 and sits atopsleeve 52 to mount the reflector between the exposed parts of theinjector and the conduit 4. Reflector 70 is retained in position by anut 74 which threads onto valve body 18.

[0053] It is desired to keep the injection pressure relatively low toprevent the fluid jet or plume from the injector from over-penetratinginto the exhaust gas stream and impinging on the sidewall of theconduit. Injection pressures within a range of 30 to 100 psi have beenfound to prevent over-penetration. An injection pressure of 67 psi ispreferred for the injector according to the invention.

[0054] However, lower injection pressures might not atomize the injectedfluid to a sufficiently fine size for effective catalytic reduction ofthe NO_(x). To assist dispersion and atomization of the fluid within theconduit and yet maintain reasonably low injection pressures, anatomization hook 64 is provided. It is an advantage of the inventionthat no secondary atomization fluid is required.

[0055] Hook 64 is mounted on the valve, preferably on the metal plate 58of heat shield 56 as seen in FIG. 2. Preferably, the hook is made ofstainless steel to withstand the corrosive environment within theexhaust conduit. Mounting the hook on the heat shield serves tothermally isolate the hook from the valve body 18. Because the hookextends into the exhaust stream, it will be hot, and being metal, itwill tend to conduct heat readily. However, by mounting the hook on theheat shield heat conducted by the hook will be blocked by the thermalgasket 60, and heat transfer from the hook to the valve body will beminimized by this preferred mounting of the hook 64.

[0056] Hook 64 has an end surface 66 which is positioned in aspaced-apart relation facing orifice 22. When the valve plunger 26 isactuated into its open position by solenoid 38, expelling fluid at apredetermined pressure from orifice 22, the fluid jet will impinge onend surface 66. This impingement will cause further atomization of thefluid. The dispersion characteristics of the fluid are a function of theshape of the end surface, which is tuned to a particular size and shapeof the exhaust stream to ensure maximum dispersion and penetration ofthe fluid without over-penetration.

[0057] An injector wherein critical valve components are directly cooledby circulating fluid according to the invention provides a component fora pollution control system which allows a corrosive and heat-sensitivereagent, such as aqueous urea, to be effectively employed to reduceNO_(x) emissions and thereby ultimately attain greater fuel efficiencywithout the adverse effects of increased undesired emissions.

What is claimed is:
 1. A method of reducing emissions of oxides ofnitrogen from a combustion process using a temperature sensitive liquidreagent injected into a stream of exhaust gases from said combustionprocess and passing said exhaust gases and said reagent through acatalytic reactor which reduces the oxides of nitrogen in the presenceof the reagent, said method comprising the steps of: providing aninjector having an orifice for atomizing said liquid reagent;positioning a portion of said injector having said orifice within saidstream of exhaust gases; cooling said injector by continuouslycirculating said reagent therethrough, thereby keeping both saidinjector and said reagent within said injector below a criticaltemperature at which said reagent will solidify; and injecting a portionof said reagent into said exhaust stream upstream of said reactor.
 2. Amethod according to claim 1, wherein said reagent is an aqueous ureasolution.
 3. A method according to claim 2, wherein said urea has aconcentration between about 25% and about 35%.
 4. A method according toclaim 1, further comprising the steps of providing a surface facing saidorifice within said exhaust gas stream, and further atomizing saidreagent injected into said exhaust gas stream by impinging said reagentonto said surface.
 5. A method according to claim 1, wherein saidcombustion process occurs within an internal combustion engine.
 6. Amethod according to claim 5, wherein said engine is a diesel engine. 7.A method according to claim 6, wherein said reagent is injected intosaid stream of exhaust gases in proportion to selected engine operatingparameters.
 8. A method of reducing emissions of oxides of nitrogen froma combustion process using a liquid reagent injected through an injectorinto a stream of exhaust gases from said combustion process, wherein atleast a portion of said injector being positioned within said stream ofexhaust gases, said method comprising the steps of: (1) continuouslycirculating said reagent through said injector to keep both saidinjector and said reagent within said injector below a criticaltemperature; (2) injecting at least a portion of said reagent throughsaid injector into said exhaust stream; and (3) passing said exhaustgases and said reagent injected therein through a catalytic reactor toreduce the oxides of nitrogen.
 9. A method according to claim 8, whereinsaid reagent is an aqueous urea solution.
 10. A method according toclaim 8, wherein said urea has a concentration between about 25% andabout 35%.
 11. A method according to claim 8, wherein said injector hasan orifice for atomizing said liquid reagent, and further comprising thesteps of providing a surface facing said orifice within said exhaust gasstream, and further atomizing said reagent injected into said exhaustgas stream by impinging said reagent onto said surface.
 12. A methodaccording to claim 8, wherein said combustion process occurs within aninternal combustion engine.
 13. A method according to claim 8, whereinsaid engine is a diesel engine.
 14. A method according to claim 8,wherein said reagent is injected into said stream of exhaust gases inproportion to selected engine operating parameters.
 15. A methodaccording to claim 9, wherein said critical temperature is between about95° C. and about 140° C.